This invention relates to a method of continuosly stretching a web of a thermoplastic sheet into an arcuate form and an apparatus for the stretching method. The thermoplastic sheet is for use as an interlayer of laminated glass, and the arcuately stretched web of the plastic sheet is to be cut into sections each of which is for use as an interlayer of a curved and laminated glass panel such as an automobile windshield.
Laminated safety glass is produced by sandwiching a thermoplastic interlayer between two glass sheets. Usually the thermoplastic interlayer is a sheet of a polyvinyl acetal represented by polyvinyl butyral.
In current automobiles and other vehicles it is prevailing to employ curved and laminated glass panels as windshields, and it is often that a curved windshield has a glare shield band extending along the top edge. Usually polyvinyl butyral sheet for use as an interlayer of laminated glass is supplied in the form of a rolled, continuous web having straight longitudinal edges. For windshields having a glare shield band, the web of polyvinyl butyral sheet has a colored band along and contiguous to one longitudinal edge of the web. Prior to the lamination operation to produce curved and laminated windshields by using a web of polyvinyl butyral sheet having a colored band, it is necessary to stretch the web of polyvinyl butyral such that the stretched web has longitudinal edges formed into arcuate configurations with different arc lengths. That is, the web needs to be arcuately stretched to a greater extent at the edge remote from the colored band than at the edge adjacent the colored band.
Several methods have been proposed for the above described stretching of a web of a thermoplastic sheet into an arcuate form. U.S. Pat. No. 3,038,208 discloses to accomplish the desired stretching by producing a temperature gradient across the width of a web of a thermoplastic sheet. That is, while the web is stretched between rolls the web is heated by dielectric or infrared heating units such that the degree of heating minimizes at the edge adjacent the colored band. At the subsequent cooling of the stretched web the plastic web has greater tendency to shrinkage at the edge heated to the lower temperature than at the opposite edge, so that the cooled web has arcuate edges with different arc lengths. In this method the arcuate shaping relies solely on the different tendencies to shrinkage at the opposite ends of the web subjected to gradient heating. However, it is difficult to precisely control the tendencies to shrinkage for obtaining an arcuate form of desired curvatures. Besides, this method pays little attention to the rate of raising the temperature of the web. If the web of thermoplastic is very rapidly heated to a temperature sufficient for stretching it is often that local shrinkage occurs in the stretched and cooled web, or longitudinal wrinkles or streaks appear on the web surfaces.
U.S. Pat. No. 3,019,475 shows a stretching method having the steps of passing a web of thermoplastic around a heated cylindrical roll and then around a heated tapered roll to thereby stretch the web into an arcuate form and then passing the heated web around a cooled tapered roll. This method does not produce a gradient of temperature across the width of the web. Therefore, the web under stretching is liable to droop in one edge region where arcuate stretching to a relatively smaller extent is intended. In this method the web is heated in two stages first by the cylindrical roll and then by the tapered roll, but according to the disclosure both of these two rolls are kept heated at about 100.degree. C. Rapid heating of the plastic web to such a high temperature renders the web tacky so that the heated web is liable to stick to the rolls or droop. U.S. Pat. No. 3,696,186 shows a stretching method having the steps of uniformly heating a web of a thermoplastic sheet, stretching the heated web into an arcuate form between a cylindrical roll and a tapered roll and cooling the stretched web while it is passing around the cylindrical roll. Since the web is heated without producing a gradient of temperature across the width of the web there is the aforementioned problem of drooping of the web at its one edge region. Besides, rapid heating of the web to a temperature of about 65.degree.-75.degree. C. often causes local shrinkage of the stretched web or results in appearance of wrinkles or streaks on the stretched web. Precedent to the stretching operation by this method the web is heated in a separate station, but this is unfavorable for the overall efficiency of the stretching process.